1. Field of the Invention
This invention relates to optical communications and more specifically to laser transmitters and to compensation for the effects caused by changes in laser current; these changes can be the result of temperature changes, aging or operating intentional changes.
2. Description of the Prior Art
When temperature increases or when a laser ages, the quantum efficiency of light emission (called also slope efficiency--.eta.) from a laser is reduced, and as a result more DC (direct current) needs to be provided to the laser to obtain the same amount of optical power from the laser.
This task of maintaining constant optical power is performed by an APC (Auto Power Control) loop that is connected externally to the laser package (which includes a laser diode and a photo-diode which receives a fraction of the laser light). As the temperature (for instance) increases, the APC loop sends more current through the laser and since the information bearing signal modulating the laser is constant, the Optical Modulating Index (OMI=m [%]) is reduced, as seen from the formula m=I.sub.mod /(I.sub.DC -I.sub.th), where I.sub.mod =modulation current and I.sub.th =threshold current. The CNR (carrier/noise ratio) at the output of the receiver that detects the optical power is proportional to m.sup.2 and therefore decreases.
The amount by which the OMI and CNR decrease depends on the laser and its slope efficiency behavior over temperature. If the slope efficiency at high temperature is for example half its value at low temperature the CNR will drop by 6 db: EQU .DELTA.(CNR)=20log(m.sub.2 /m.sub.1)
It has been found that slope efficiency falls by a factor of 1.5 to 2 at 85.degree. C. compared to its value at 25.degree. C., and the corresponding drop in CNR is 3.5 to 6 db.
Several prior solutions to this problem have been proposed, as follows:
1. External temperature sensing and temperature control.
This solution does not compensate for slope efficiency changes over temperature but prevents (or minimizes) them by controlling the temperature of the laser. The way the temperature is controlled is as follows: a temperature sensor (i.e. thermistor) is located close to the laser such that its temperature is as close as possible to the laser temperature. A power resistor is located close to the laser and thermally conductive epoxy couples the thermistor, the laser and the power resistor.
A circuit is connected to these elements such that whenever their temperature goes below a certain threshold, a driver sends current to the power resistor and heats it up, the conductive epoxy transfers the heat to the laser and the thermistor and they heat up also. This process continues until the temperature of the thermistor (and therefore the laser) is slightly above the threshold. This method is good when the ambient temperature (and therefore the laser temperature) is below the threshold temperature because the circuit only needs to send power to the laser and thereby heats it up. The problem is when the ambient temperature is above the threshold temperature, in this case the laser is warmer than it should be and it needs to be cooled down. The problem is that the system cannot cool down at all. The solution to this shortcoming is to make the threshold temperature high enough such that it is above the highest possible ambient temperature. This ensures that always one adds power to the laser and there is no need to cool it down.
There are three problems with this solution:
a) The laser operates constantly at high temperature so its life expectancy is shorter. PA1 b) The laser is operated at fixed slope efficiency but a low one, therefore more DC current and more modulating current are needed. PA1 c) Lasers suffer from some other non-idealities at high temperature (e.g. more distortion, more noise) and since they are operated according to this solution at high temperature, the specifications might be adversely affected.
2. Cooled laser with internal thermo-electric cooler
This uses a laser mounted on a thermo-electric cooler that either can heat up or cool down the laser to a desired fixed temperature regardless the ambient temperature. The result is that the slope efficiency is fixed and so are OMI and CNR.
The disadvantage of this solution is that the laser with the cooler is much more expensive and the external circuitry is also more complicated and expensive.